Electric heating circuit for electric heating devices



-23,1 58 G. R. DOLAN ETAL 2,866,067

ELECTRIC HEATING CIRCUIT FOR ELECTRIC HEATING DEVICES Filed Feb. 11, 1957 j? 672 $07 5: 507*4072 1? iazza 72 (arrel y%.s W fad/15% ELECTRIC HEATING CIRCUIT FOR ELECTRIC HEATING DEVICES Gordon R. Dolan, Des Plaines, Ill., and Carrel I-I. Dykes,

Bay Springs, Miss., assignors to Northern Electric Company, Chicago, Ill., a corporation of Delaware Application February 11, 1957, Serial No. 639,413

6 Claims. Cl. 219-20 iting circuit associated therewith which prevents the heated blanket, sheet, pad or the like from reaching excessively high temperatures.

As a safety feature in the use of electric blankets, electric sheets, electric pads or the like, circuits have heretofore been devised for disconnecting the heating elements of the device from the source of current when United States Pa n the heat of the device becomes excessive or approaches heating circuit it will automatically periodically tend to reset itself until the cause of the overheating is removed at which time the resetting becomes effective.

Another important object of the present invention resides in a temperature limiting circuit which is so devised that in the event of failure of any of the components thereof the disconnection of the heating element' from the source of current is brought about and the possibility of dangerous overheating is eliminated.

Other important features and advantages of the invention will be apparent from the accompanying drawing and following detailed description.

The single figure comprising the drawing is a schematic wiring circuit embodying the invention.

In the drawing, terminals 1 and 2 comprise connections to a source of current such as the usual house current. A conventional oif and on control switch 3 is connected in conductor 4 which, in turn, is connected to terminal 1. A series resistance 5 of predetermined wattage and a series resistance 6 of a higher wattage are also connected with conductor 4, said conductor also having a conventional bimetallic thermostatic switch 7 connected therewith in series with the resistances 5 and 6.

As will be hereinafter more fully described, resistances 5 and 6 and switch 7 are carried in the usual control box (not shown) employed with electric blankets, electric sheets and the like, that is, a box which is separate from and remote from the heated blanket, sheet or the like.

The conductor 4, at its end opposite terminal 1 is con- .nected to common junction terminal 8 of a Wheatstone bridge circuit 9, and a conductor 10 connects terminal 2 to the common junction terminal 11 of the bridge 9. Thus, bridge 9 is connected across the source of current supply through switches 3 and 7 and through one or both of resistances 5 and 6, as will be hereinafter more fully described.

2,866,067 en ed e 23,125

The bridge 9 comprises resistance elements or arms 12 and 13 connected in series between terminals 8 and 11 and resistance elements or arms 14 and 15 are similarly connected in series between terminals 8 and 11 and in parallel with resistance elements 12 and 13. 'A conductor 16 is connected at one end to the common junction point 17 of resistance elements or arms 12 and 13 and a conductor 18 is connected to the common junction point 19 of resistance elements or arms 14 and 15. Conductor 16 connects through a rectifier 20 to one end of a relay coil 21 and conductor 18 connects with the opposite end of said coil.

As will be hereinafter more fully described, resistance elements 12, 13, 14 and 15 comprise the heating elements of the blanket, sheet, pad or the like.

An armature 22 is mova bly associated with relay coil 21, said armature cooperating with switch points 23 and 24 to connect or open short circuiting conductors 25. The relay switch 26 thus formed is of the normally open type and is adapted to be closed when a current of predetermined value passes through coil 21.

As a feature of the present invention resistance element 12 is constructed of a material which has a relatively low temperature coeflicient of resistance, that is, a material whose resistance varies at a relatively low rate with temperature. Resistance element 13, on the other hand, is constructed of a material which has a high temperature coefiicient of resistance, that is, its resistance varies at a relatively high rate with temperature change.

Conversely, resistance element 14, connected to the common terminal 8, has a relatively high temperature coefficient of resistance and resistance element 15 connected to the common terminal 11 has a relatively low temperature coefficient of resistance.

The materials comprising the resistance elements 12, 13, 14 and 15 are correlated, with respect to their normal resistances at a predetermined temperature, so that within a predetermined temperature range voltages ranging within a predetermined range will appear across terminals 17 and 19. That is, the constants of the resistance elements are so selected that within a predetermined temperature range, the bridge 9 will be unbalanced to a predetermined degree. Thus, within said predetermined temperature range a predetermined current will flow through relay coil 21, the constants of the coil being such as to release armature 22 when a predetermined minimum current flows through the coil.

In view of the fact that such devices as electric blankets, electric sheets, electric pads or the like, are usually connected to a source of alternating current, the, current flowing through coil 21 due to the voltage set up by the unbalance of bridge 9 is rectified by the rectifier 29. T hus. direct current will be fed to coil 21.

The hereinbefore described operation takes place by virtue of the different temperature c'oefiicients of resistance of the arms of the bridge. For instance, it will be noted that as the heat of the resistance elements increases, the resistance of elements 13 and 14 will increase at a greater rate than the resistance of elements 12 and 15. Hence, if bridge 9 is normally unbalanced, which is the preferred condition, as the heating increases, the bridge tends toward the balanced condition. It can be readily appreciated, therefore, that the bridge may be so set that as a limiting maximum heating is approached, the current through relay coil 21 approaches a minimum and eventually armature 22 will be released thereby eliminating the short circuit 25 and placing resistance 6 in series with the bridge 9 and the source of current. 1

In the normal operation of the device, thermostatic .switch 7 will be closed, that is, it is closed in the cold or t oo er es- I he s flsl P931 installsfore mentioned, resistance is in close proximity to switch 7 so as to be in heat-transfer relationship with said switch. When control switch 3 is turned on, the resistance elements 12, 13, 14 and will be energized and inasmuch as the bridge 9 is normally unbalanced, relay switch 26 will be closed. Hence, only resistance 5 will be in series with the bridge 9 and the source of current. As the temperature of elements 12, 13, 14 and 15 rises the temperature of resistance 5 rises until the temperature of resistance 5 actuates switch 7. The latter switch then opens momentarily but closes shortly thereafter and thereafter cyclically opens and closes to maintain the device at a substantially constant temperature. This is the normal operation of the device. Of course, as the temperature of the elements 12, 13, 14 and 15v increases the current through coil 21 decreases but such. decrease is not sufficient, at the cycling phase of resistance 5 and switch 7, to open relay switch 26. Hence, switch '1' under the control of resistance 5 continues to open and close establishing a condition of equilibrium.

If now, for some reason, such as, lack of normal radiation, the temperature of the blanket, sheet or pad increases excessively, resistance 5 and switch 7 at the time functioning in their normal manner, the bridge 9 tends toward a balanced condition and eventually the current through relay coil 21 reaches the preset minimum at which relay switch 26 opens. At this phase. of operation, resistance 6 is placed in circuit. Resistance 6 is also in close proximity to switch 7 and is of greater wattage than resistance 5 and, hence, the heating of resistance 6 dominates in controlling switch 7. Hence, in the situation described, resistance 6 will open switch 7 and the heating elements 12, 13, 14 and 15 will be deenergized. When the blanket, sheet or pad again cools resistance 6 also cools and switch 7 again closes.

If the condition which caused the overheating is re removed, bridge 9 will remain unbalanced sufficiently to energize relay coil 21 and short out resistance 6 and therefore resistance 5 will resume its normal control of switch 7. If, however, the condition. which caused the overheating still persists, resistance 6 maintains control ofswitch 7, in the manner hereinbefore described, and the maximum preset temperature of the blanket will never be exceeded.

It can readily be understood that in this fashion, the constants of the circuit elements may be so selected that for normal operation, resistance 6 will remain inactive and an equilibrium will be established by resistance 5 and switch 7, and that when an abnormal heating condition is encountered, resistance 6 will be inserted automatically in circuit and function to limit the maximum temperature to which the device may be raised.

It will be noted that current must flow through relay coil for resistance 6 to be held out of circuit. Hence, if coil 21 is opened or shorted, armature 22 will return to its normal open position, thereby placing resistance 6 in circuit. With resistance 6 in circuit excessive temperature cannot be reached. It will also be noted that if either of the resistances 5 or 6 burn out, the entire devicewill be deenergized or if any of the resistance elements 12, 13, 14 and 15 open, coil 21 will be deenergized and re sistance 6 will be again placed in circuit to exercise its characteristic limiting function.

Thus, normal failure of any of the components will not permit an excessive rise of temperature in the heated device.

We claim as our invention:

1. An electrical heating circuit having a temperature limiting control which comprises, four electrical heating elements connected as respective arms in a Wheatstone bridge circuit, adjacent arms of said bridge comprising heating elements respectively having relatively high and relatively low thermal coefiicients of resistance, a current supply for said bridge, a normally closed' thermostatically operated switch connected inseries withthe current supply to said bridge, a resistance of relatively low wattage connected in series with said current supply and disposed in heat transfer relationship to said thermostatically operated switch to open said switch when the said resistance reaches a predetermined temperature, a second resistance of relatively high wattage connected in series with said current supply and in heat transfer relationship to said thermostatically operated switch, a short circuit for said relatively high wattage resistance, means connected across the common junctures of op posite arms of said bridge for opening and closing said short circuit in response to a predetermined unbalanced condition of said bridge.

2. An electrical heating circuit having a temperature limiting control which comprises, four electrical heating elements connected as respective arms in a Wheatstone bridge circuit, adjacent arms of said bridge comprising heating eleemnts respectively having relatively high and relatively low thermal coefficients of resistance, a current supply for said bridge, a normally closed thermostatically operated switch connected in series with the current supply to said bridge, a resistance of relatively low wattage connected in series with said current supply and disposed in heat transfer relationship to said thermostatically operated switch to open said switch when the said resistance reaches a predetermined temperature, a second resistance of relatively high wattage connected in series with said current supply and in heat transfer relationship to said-thermostatically operated switch, a short circuit for said relatively high wattage resistance, means connected across the common junctures of opposite arms of said bridge. for closing said short circuit in response to a predetermined voltage across said common junctures and for opening said short circuit in response to a voltage below said predetermined voltage across said common junctures.

3. An electrical heating circuit having a temperature limiting control which comprises, four electrical heating elements connected as respective arms in a Wheatstone bridge circuit, adjacent arms of said bridge comprising heating elements respectively having relatively high and relatively low thermal coefficients of resistance, a normally open relay switch having its control coil connected across the common junctures of opposite arms of said bridge, a current supply for said bridge, a normally closed thermostatically operated switch connected in series with the current supply to said bridge, a resistance of relatively low wattage connected in series with said current supply and disposed in heat transfer relationship to said thermostatically operated switch to open said switch when the said resistance reaches a predetermined temperature, a second resistance of relatively high wattage connected in series with said current supply and in heat transfer relationship to said thermostatically operated switch, a short circuit for said relatively high wattage resistance which includes said relay switch whereby above a predetermined minimum unbalance of said bridge said relay switch is maintained closed and below said predetermined minimum unbalance of the bridge said relay switch is maintained open to heat said high wattage resistance to open said thermostatic switch.

4. An electrical heating circuit having a temperature limiting control which comprises, four electrical heating elements connected as respective arms in a Wheatstone bridge circuit, adjacent arms of said bridge comprising heating elements respectively having relatively high and relatively low thermal coefiicients of resistance, a normally open relay switch having its control coil connected across the common junctures of opposite arms of said bridge, a current supply for said bridge, 21 normally closed thermostatically operated switch connected in series with the current supply to said bridge, a resistance of relatively low wattageconneeted in series with said thermostatically operated switch and disposed in heat transfer relationship to said thermostatically operated switch to open said switch when the said resistance reaches a predetermined temperature, a second resistance of relatively high wattage connected in series with said thermostatically operated switch and in heat transfer relationship to said thermostatically operated switch, a short circuit for said relatively high wattage resistance which includes said relay switch whereby above a predetermined minimum unbalance of said bridge said relay switch is maintained closed and below said predetermined minimum unbalance of the bridge said relay switch is maintained open to heat said high wattage resistance to open said thermostatic switch.

5. An electrical heating circuit having a temperature limiting control which comprises, four electrical heating elements connected as respective arms in a Wheatstone bridge circuit, the resistance of said heating elements being such that the bridge is unbalanced when cold, adjacent arms of said bridge comprising heating elements respectively having relatively high and relatively low thermal coefficients of resistance whereby an increase in temperature urges said bridge toward balanced condition, a normally open relay switch having its control coil connected across the common junctures of opposite arms of said bridge, a current supply for said bridge, a normally closed thermostatically operated switch connected in series with the current supply to said bridge, a resistance of relatively low wattage connected in series with said current supply and disposed in heat transfer relationship to said thermostatically operated switch to open said switch when the said resistance reaches a predetermined temperature, a second resistance of relatively high wattage connected in series with said current supply and in heat transfer relationship to said thermostatically operated switch, a short circuit for said relatively high wattage resistance which includes said relay switch whereby above a predetermined minimum unbalance of said bridge said relay switch is maintained closed and below said predetermined minimum unbalance of the bridge said relay switch is maintained open to heat said high wattage resistance to open said thermostatic switch.

6. An electrical heating circuit having a temperature limiting control which comprises, four electrical heating elements connected as respective arms in a Wheatstone bridge circuit, adjacent arms of said bridge comprising heating elements respectively having relatively high and relatively low thermal coeflicients of resistance, the relative values of the resistances of said pairs of arms being such as to result in a bridge unbalance when said heating elements are relatively cold and progressing toward bridgebalance as the temperature increases, a current supply for said bridge, a normally closed thermostatically operated switch connected in series with the current supply to said bridge, a resistance of relatively low wattage connected in series with said current supply and disposed in heat transfer relationship to said thermostatically operated switch to open said switch when the said resistance reaches a predetermined temperature, a second resistance of relatively high wattage connected in series with said current supply and in heat transfer relationship to said thermostatically operated switch, a short circuit for said relatively high wattage resistance, means connected across the common junctures of opposite arms of said bridge for opening and closing said short circuit in response to a predetermined unbalance condition of said bridge.

References Cited in the file of this patent UNITED STATES PATENTS 2,093,745 Wastell Sept. 21, 1937 2,510,040 Rudahl May 30, 1950 2,522,753 Drobisch Sept. 19, 1950 2,588,926 Holmes Mar. 11, 1952 2,779,852 Weber Jan. 29, 1957 2,801,317 Goldmuntz et al July 30, 1957 2,834,862 Meyers May 13, 1958 FOREIGN PATENTS 144,669 Great Britain Sept. 9, 1921 

